<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .a{
      height: 100px;
      background-color: chartreuse;
      margin-top: 50px;
      /* border-top: 1px solid transparent; */
      overflow: scroll;
    }
    .b{
      height: 150px;
      background-color: crimson;
      margin-top: 200px;
    }
  </style>
</head>
<body>
  <div class="a">
    <div class="b">
    </div>
  </div>
</body>
</html>